Optically pumped far infrared (FIR) gas lasers are most widely used in interferometers and polarimeters for plasma diagnostics. The long wavelength of these lasers means large phase signals to be detected and insensitiveness to mechanical vibrations [1, 2]. High gain and high FIR laser output power is required so that the laser beam can be split into multiple beams for simultaneous multi-channel measurements. A typical FIR laser resonator consists of a flat mirror, a dielectric waveguide, and an output coupler, as shown in FIG. 1. The resonator is filled with molecular gas chosen based on the desired output laser wavelength. The molecules are excited to the higher vibrational energy level by absorbing infrared pump laser photons. The transition to a lower rotational energy sub-level within the same vibrational energy level branch results in the longer wavelength FIR laser output. CO2 lasers are most frequently used as the pump source due to their high power and richness in wavelength which can be tuned to match the resonant absorption of various working gas molecules. The flat mirror is coated to have near unity reflection coefficient and has a small hole (usually off center, 3-5 mm in diameter) for pump laser beam injection. It is sometimes referred to as the rear mirror as it is at the opposite side of the output coupler, where the FIR laser beam exits. The output coupler has finite transmission (coupling) coefficient so that laser power can be coupled out. This is the most critical component of FIR lasers, as it will determine the gain, output power, and output laser beam quality of the lasers, as well as how conveniently one can operate the laser. The embodiments described herein include an innovative design that achieved the highest gain and laser output power when working with the formic acid (HCOOH) vapor and a FIR wavelength of 433 μm, at the mean time significantly improved the flexibility in terms of laser tuning and operation. The laser embodiments can be easily adapted to operate at other FIR laser wavelengths.
For optically pumped FIR lasers as shown in FIG. 1, many works have been published on various output couplers, which are critical in determining the laser gain, power, and output beam quality. The simplest output coupler is a mirror with a small clear hole at the center. The pump laser beam will leak through the hole leading to reduced FIR laser gain and power. A hybrid hole output coupler, as show in FIG. 2(a), prevents the pump beam leakage by coating the hole with dielectric layers which have high reflectivity for the pump laser wavelength. The dielectric layer is thin comparing to the FIR wavelengths, showing negligible reflection. High resistivity silicon or germanium is used as the substrate material for the dielectric and annular gold coatings. These hybrid hole couplers can achieve higher FIR laser power [10, 11], but have the disadvantages of fast beam divergence and unstable transverse mode quality. Silicon substrate metal mesh-dielectric hybrid couplers, as shown in FIG. 2(b) [12], yield much better output beam quality, but are narrow band and lack the flexibility for tuning the coupling coefficient. An improved Fabry-Pérot coupler [13], as shown in FIG. 2(c), comprises a quartz étalon and metal mesh can be optimized by tuning the spacing between the quartz plate and the mesh. However, it is enclosed inside the vacuum chamber, making it very inconvenient for optical alignment and laser tuning.
The above mentioned output couplers also commonly serve as the reflector for the CO2 pump laser beam. Due to the finite length of the FIR laser cavity, the absorption of the pump laser requires multiple passes. For example, the absorption coefficient of the 9R20 line of CO2 laser in the formic acid (HCOOH) vapor with a pressure of ˜400 mTorr is 0.36 m−1 [14], and four (4) passes are required to absorb 90% of the pump power in a FIR cavity length of 1.5 m. As the pump laser is usually injected via an off-axis hole in the rear mirror, the normal of the pump beam reflector needs to be at a small angle with respect to the waveguide axis for optimum excitation. On the other hand, FIR laser alignment requires that the reflecting surface of the output couplers be perpendicular to the waveguide axis. The excitation of the laser is compromised by the FIR laser alignment as a result.
In light of the foregoing, it is, therefore, desirable to provide an improved optically pumped FIR laser.